Deterministic Program Clock Reference Re-stamping for Synchronous Bit Rate Adaptation based on Mega-frame Initialization Packet

ABSTRACT

A system and processes create a transport stream carrying several encapsulated contents, watermarks, and other ancillary data. The transport stream is re-multiplexed while keeping synchronous and deterministic operation. Synchronous deterministic operation allow each derived transport stream to be broadcasted in Single Frequency Networks, as all re-multiplexers have the same signal at the input and selecting the same content generates exactly the same stream in multiple locations.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. § 119(e) of U.S.Provisional Application No. 60/929,169 titled “Deterministic ProgramClock Reference Re-stamping for Synchronous Bit Rate Adaptation based onMega-frame Initialization Packet” filed on Jun. 15, 2007 in the U.S.Patent and Trademark Office, and of U.S. Provisional Application No.60/929,170 titled “Mega-frame Initialization Packet Generation andSynchronous Re-generation with Bit Rate Adaptation for Single FrequencyNetworks with Multiple Modulation Schemes” filed on Jun. 15, 2007 in theU.S. Patent and Trademark Office, of which the entire disclosures ofboth applications are herein incorporated by reference in their entiretyfor all purposes.

TECHNICAL FIELD

The following description relates generally to a system useful in SingleFrequency Networks, and in particular to a system useful in SingleFrequency Networks for DVB-T/H complying with EuropeanTelecommunications Standards Institute (ETSI) EN 300 744 and ETS 101191, and related international standards. The systems and methoddescribed herein are also applicable in any other system that requiresbit rate adaptation with PCR re-stamping and delivers a deterministicand identical bit stream at all participating equipment.

BACKGROUND

European Telecommunications Standards Institute—European Standard ETSIEN 300 744 describes a broadcasting system for terrestrial distributionbased on Orthogonal Frequency-Division Multiplexing COFDM (a digitalmulti-carrier modulation scheme that uses a large number ofclosely-spaced orthogonal subcarriers) and MPEG2 (encoding and transportdata system, for example, as defined by ISO/IEC 13818-1.MPEG)technologies. Application of the European Telecommunications StandardsInstitute—Technical Specification ETSI ETS 101 191 extend the user ofthis system to Single Frequency Networks (SFN), for example, as definedby ETS 101 191 and in Advanced Television Systems Committee (ATSC) A110.

The extension to SFN is accomplished by periodically adding one packetincluding synchronization time stamps and modulation parametersinformation to the transport stream (e.g., as defined by ISO 138181-1)of all transmitters broadcasting in a SFN area. The packet that is addedto the transport stream is a Mega-frame Initialization Packet (MIP). TheMIP is an MPEG2 transport stream compliant packet that carriesmega-frame and modulation parameters, for example, as defined by ETS 101191.

SUMMARY

In one general aspect, a method of providing a fixed and deterministicbit rate adaptation for an output transport stream is described below.The method includes receiving an input transport stream of packets;deriving output timing from the input transport stream; determining aninput time stamp for each packet of the input transport stream;determining a corresponding output timestamp for each outgoing packet ofthe output transport stream; determining the difference between theinput timestamp and the output timestamp; extracting a program clockreference (PCR) for the output transport stream; adding the determineddifference to the PCR; and adding the PCR to the output transportstream.

The method also may includes receiving the input stream of packetsincludes a time mark packet; determining a period from the time mark;determining the input time stamp and output time stamp for each packetwithin the period; and resetting the timestamps at the end of eachperiod.

The time mark may be a mega-frame initialization packet (MIP) and theperiod is a mega-frame. The time mark also may be a mega-frameinitialization packet (MIP) and the period is an extra long mega-frame.

The method may further include the output transport stream from theinput transport stream packets by inserting or eliminating null packetsto perform rate adaptation based on the output timing.

The time base sampling frequency may be 34,292,160 MHz for a referenceclock of 27 MHz and the difference is divided by 1,270,080.

The time base sampling frequency may be 5,670 MHz for a reference clockof 8 MHz and the difference is divided by 210.

In another general aspect, a system for providing a fixed anddeterministic bit rate adaptation for an output transport stream isdescribed below. The system may include an input to receive an inputtransport stream of packets; a timer to provide output timing derivedfrom the input transport stream; a counter to determine an input timestamp for each packet of the input transport stream; a counter todetermine a corresponding output timestamp for each outgoing packet ofthe output transport stream; a processor to determine the differencebetween the input timestamp and the output timestamp, extract a programclock reference (PCR) for the output transport stream; add thedetermined difference to the PCR; and add the PCR to the outputtransport stream; and an output to output the transport stream accordingto the derived output timing.

The input stream may include a time mark packet used to determine aperiod, the input time stamp and output time stamp are determined foreach packet within the period; and the timestamps are reset at the endof each period. The time mark may be a mega-frame initialization packet(MIP) and the period is a mega-frame. The time mark also may be amega-frame initialization packet (MIP) and the period is an extra longmega-frame.

The system also may include a composer to create the output transportstream from the input transport stream packets by inserting oreliminating null packets to perform rate adaptation based on the outputtiming.

The time base sampling frequency may be 34,292,160 MHz for a referenceclock of 27 MHz and the difference is divided by 1,270,080. The timebase sampling frequency also may be 5,670 MHz for a reference clock of 8MHz and the difference is divided by 210. Other features will beapparent from the detailed description, drawings, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exemplary block diagram of a system for single frequencynetworks.

DETAILED DESCRIPTION

Transport streams suitable for DVB SFN systems are defined by ETSIstandard. The standard defines a bit rate and a synchronization packet(MIP) for each set of four parameters: channel Band Width (BW), GuardTime interval (GT), Constellation (CON), and Forward Error Correctionratio (FEC). The SFN transport stream must be the same (i.e., all bitsin the stream must be equal bit after bit) at all transmittersbroadcasting in an SFN area. However, some examples where the parametersrequire changing are as follows:

-   -   receiving an 8 MHz BW incoming signal which must be broadcast to        an area where a 7 MHz channel is required (e.g., often the case        in Italy and Germany where channels have mixed BW);    -   receiving an ¼ GT+x/xFEC incoming signal (e.g., where ¼ GT is        used in a main transmitter for optimum SFN performance) which        must be broadcast to an area where a smaller GT (e.g., 1/32 GT,        used in small area coverage) may be used and a more robust        y/yFEC provides increased signal protection for a lower power        transmitter; and    -   receiving a 1/32 GT+x/xFEC incoming signal (e.g., 1/32 GT is        used in main transmitter where SFN operation is not required)        which must be broadcast to SFN mode where an improved GT (e.g.,        ⅛ GT, used in medium area SFN coverage) may be used and a less        robust y/yFEC provides a similar payload.

In these examples, when the transport stream changes from one set ofparameters to another, the change in the transport stream bit rate, MIP,and PCR information must be determined. In a conventional system, thechange in bit rate is made in a random way by inserting or subtractingnull packets from an incoming transport stream and inserting a new MIPpacket in a new random position. However, this random process does notguarantee that the bit stream at two different locations will be exactlythe same in each bit (i.e., bit exact). In addition, the PCR re-stampingis typically made by modifying PCR information based on time-of-arrivaland time-of-departure difference of transport stream packets. However;within the network there may be small differences due to jitter andequipment delays. Even the time-of-arrival itself is not a finitequantity. Furthermore, these small differences in PCR correction valuesare normally made between different locations. As a result, differentsystems are not able to generate the same bit sequence. The followingdescription provides a process and system for determining PCRre-stamping that produces a bit stream that is bit exact at anyparticipating system, even in two different locations.

FIG. 1 shows an example 100 of a basic block diagram of a system 100 fordeterministic program clock reference re-stamping for synchronous bitrate adaptation using an MIP. The input signal may be any a transportstream 101, for example, as provided for in the ISO/IEC 13818-1standard. The transport stream may include packets include packetsgrouped into a mega-frame, which is a group of a integer number of TSpackets, for example, as specified by ETS-101 191. The transport stream101 is supplied to detector 105, time stamp generator 110, and memory115.

The detector 105 processes the transport stream 101 to extract receivedmega frame information, such as the MIP parameters and related transportstream parameters, mega frame time boundaries, and, when available,extra-long mega frame time boundaries of the received tr. The detector105 provides the receive time frame boundaries to detector 107.

The time stamp generator 110 computes input time stamps or an input timebase for each incoming packet of the transport stream. The time basecount is incremented linearly at every input packet. The count is resetby the signal Reset TB 108 provided every time at the end of a MegaFrame or of an Extra-Long Mega Frame.

The memory 115 stores each input transport stream packet with a relatedtime stamp computed by the time stamp generator 110. In one example, thememory may be implemented using a First In-First Out (FIFO) memory,where the first data written in are the first data read.

The transport stream also is provided to output timing generator 120.The output timing generator 120 generates an output timing signal basedon the input transport stream bit rate. Since input and output transportstreams are linked by a fixed ratio, all pulses and timing signalsrequired to build the output transport stream are derived from the inputtransport stream bit rate, as is known to those skilled in the art.

Processing block 125 computes the transmit mega-frame information forthe outgoing transport stream, such as the mega frame time boundaries,and, when available, extra-long mega frame time boundaries. The transmittime frame boundaries are provided to detector 107.

The Detector 107 detects the end/start of each mega frame in thetransport steam, or, in the case that the input and the output megaframe do not have the same length, detects the end of an extra-long megaframe. When the detector 107 detects the end of a mega frame orextra-long mega frame, the detector generates the signal reset TB 108.

The output time stamp generator 130 computes the output time stamps ortime base for each outgoing packet. The output time base count isincremented linearly at every output packet. The base count is reset bythe signal reset TB 108 provided at the end of every mega frame orextra-long mega frame.

Block 135 determines the difference between the output time stampprovided by generator 130 and the input time stamp TS_TB 137 computed bythe input time stamp generator 110 that is stored with each associatedpacket in the FIFO memory 115. The received packets are output from theFIFO memory 115 to the transport stream composer 140. The transportstream composer 140 is provided with the output timing and performs bitrate adaptation of the transport stream by determining whether toeliminate or insert null packets in the transport stream. The transportstream is provided to the extractor 145.

Extractor 145 detects if any of the packets of the transport stream havean adaptation field with a PCR. If a PCR is detected, the extractor 145extracts the PCR and sets a flag EN_PCR_INSERT 147. Any extracted PCR isprovided to adder 150 which adds the time difference generated by block135 to the extracted PCR to provide the recalibrated PCR. Inserter 155re-inserts any recalibrated PCR into the transport stream whenEN_PCR_INSERT flag is set. The adapted transport stream is delivered tooutput connector TS OUTPUT 160.

According to the systems and methods described herein, the input andoutput transport streams have a fixed and deterministic bit rate, evenif ruled by a fractional ratio, and an input transport stream carries atime mark, such as the MIP packet, which may be used to identify a timeperiod. The time period allows determination of a periodical startingpoint common to input and output transport streams by indicating whereto reset the PCR adaptation process. To have comparable exact input andoutput time stamps at any possible input and output transport streamrate identified in EN 300 744, the time base sampling frequency of34,292,160 MHz may be used as a virtual reference clock.

Table 1 shows a packets period expressed in cycles of time base samplingfrequency for every combination of guard time (e.g. as defined by EN 300744), constellation, Forward Error Correction (FER) in an 8 MHz channel.It may be noted that all values are integers and exact.

TABLE 1 ¼ ⅛ 1/16 1/32 QPSK ½ 10363852800 9327467520 88092748808550178560 ⅔ 7772889600 6995600640 6606956160 6412633920 ¾ 69092352006218311680 5872849920 5700119040 ⅚ 6218311680 5596480512 52855649285130107136 ⅞ 5922201600 5329981440 5033871360 4885816320 16 QAM ½5181926400 4663733760 4404637440 4275089280 ⅔ 3886444800 34978003203303478080 3206316960 ¾ 3454617600 3109155840 2936424960 2850059520 ⅚3109155840 2798240256 2642782464 2565053568 ⅞ 2961100800 26649907202516935680 2442908160 64 QAM ½ 3454617600 3109155840 29364249602850059520 ⅔ 2590963200 2331866880 2202318720 2137544640 ¾ 23030784002072770560 1957616640 1900039680 ⅚ 2072770560 1865493504 17618549761710035712 ⅞ 1974067200 1776660480 1677957120 1628605440Stamping with an exact value each input or output packet inside thereference period, the period of one Mega Frame or one Extra-Long MegaFrame allows the difference between the arrival time and the departuretime of the transport stream to be determined. In addition, thecomputation is independent of any eventual jitter found in an incomingtransport stream.

MPEG2 systems are based on an MPEG reference clock (a reference clock isused by transport stream system target decoder, for example, as definedby ISO/IEC 13818-1). The MPEG reference clock has a frequency of 27 MHz.To express a computed delay in the number of the reference clock, thedifference is divided by 1,270,080, which is the ratio between the timebase reference clock and the MPEG reference Clock. Addition of thecomputed time difference to the PCR value is performed using dualmodulus arithmetic, as described in IS013818-1. The time base samplingfrequency is only one example, and that many others may be computedaccording to the teachings herein.

For example, much lower frequencies my be found if only a subset ofpossible combinations is required like that shown in Table 2, which isbase on the time base sampling frequency of 5,670 MHz. In this example,the flexibility is reduced compared to that of the first value, but forapplications dealing with channel bandwidth of only 8 MHz it issufficient. For this table and time base sampling frequency, thecomputed delay difference is divided by 210 to get the value added tothe PCR.

TABLE 2 ¼ ⅛ 1/16 1/32 QPSK ½ 1713600 1542240 1456560 1413720 ⅔ 12852001156680 1092420 1060290 ¾ 1142400 1028160 971040 942480 ⅚ 1028160 925344873936 848232 ⅞ 979200 881280 832320 807840 16 QAM ½ 856800 771120728280 706860 ⅔ 642600 578340 546210 530145 ¾ 571200 514080 485520471240 ⅚ 514080 462672 436968 424116 ⅞ 489600 440640 416160 403920 64QAM ½ 571200 514080 485520 471240 ⅔ 428400 385560 364140 353430 ¾ 380800342720 323680 314160 ⅚ 342720 308448 291312 282744 ⅞ 326400 293760277440 269280

This process gives examples of a way to implement a deterministicprogram clock reference re-stamping in Digital Video Broadcasting forHandheld Mobile/Terrestrial Television DVB-T/H (e.g., as defined in EN300 744 and in TR102 377DVB-T/H) systems where mega-frame initializationpacket or extra-long mega frame watermarking is present. The processesdescribed above guarantee that any equipment having the same signal atthe input and having the same output bit rate, and generates exactly thesame stream (bit exact) in multiple locations. As a result, the systemand methods described herein allow the use of the embedded Mega-frameInitialization Packet to achieve a high precision and deterministic PCRre-stamping that can be applied at the input of each transmitter of anSFN network, thereby maintaining the SFN requirements of identical bitstream at every transmitter and being fully compliant withabove-mentioned standards.

A number of exemplary implementations have been described. Nevertheless,it will be understood that various modifications may be made. Forexample, suitable results may be achieved if the steps of describedtechniques are performed in a different order and/or if components in adescribed components, architecture, or devices are combined in adifferent manner and/or replaced or supplemented by other components.Accordingly, other implementations are within the scope of the followingclaims.

1. A method of providing a fixed and deterministic bit rate adaptationfor an output transport stream, the method comprising: receiving aninput transport stream of packets; deriving output timing from the inputtransport stream; determining an input time stamp for each packet of theinput transport stream; determining a corresponding output timestamp foreach outgoing packet of the output transport stream; determining thedifference between the input timestamp and the output timestamp;extracting a program clock reference (PCR) for the output transportstream; adding the determined difference to the PCR; and adding the PCRto the output transport stream.
 2. The method of claim 1 furthercomprising: receiving the input stream of packets includes a time markpacket; determining a period from the time mark; determining the inputtime stamp and output time stamp for each packet within the period; andresetting the timestamps at the end of each period.
 3. The method ofclaim 2 wherein the time mark is a mega-frame initialization packet(MIP) and the period is a mega-frame.
 4. The method of claim 2 whereinthe time mark is a mega-frame initialization packet (MIP) and the periodis an extra long mega-frame.
 5. The method of claim 1 further comprisingcomposing the output transport stream from the input transport streampackets by inserting or eliminating null packets to perform rateadaptation based on the output timing.
 6. The method of claim 2 whereinthe time base sampling frequency is 34,292,160 MHz for a reference clockof 27 MHz and the difference is divided by 1,270,080.
 7. The method ofclaim 2 wherein the time base sampling frequency is 5,670 MHz for areference clock of 8 MHz and the difference is divided by
 210. 8. Asystem for providing a fixed and deterministic bit rate adaptation foran output transport stream, the system comprising: an input to receivean input transport stream of packets; a timer to provide output timingderived from the input transport stream; a counter to determine an inputtime stamp for each packet of the input transport stream; a counter todetermine a corresponding output timestamp for each outgoing packet ofthe output transport stream; a processor to determine the differencebetween the input timestamp and the output timestamp, extract a programclock reference (PCR) for the output transport stream; add thedetermined difference to the PCR; and add the PCR to the outputtransport stream; and an output to output the transport stream accordingto the derived output timing.
 9. The system claim 8 wherein the inputstream includes a time mark packet used to determine a period, the inputtime stamp and output time stamp are determined for each packet withinthe period; and the timestamps are reset at the end of each period. 10.The system of claim 9 wherein the time mark is a mega-frameinitialization packet (MIP) and the period is a mega-frame.
 11. Thesystem of claim 9 wherein the time mark is a mega-frame initializationpacket (MIP) and the period is an extra long mega-frame.
 12. The systemof claim 8 further comprising a composer to create the output transportstream from the input transport stream packets by inserting oreliminating null packets to perform rate adaptation based on the outputtiming.
 13. The system of claim 9 wherein the time base samplingfrequency is 34,292,160 MHz for a reference clock of 27 MHz and thedifference is divided by 1,270,080.
 14. The system of claim 9 whereinthe time base sampling frequency is 5,670 MHz for a reference clock of 8MHz and the difference is divided by 210.